1. Field of the Invention
The present invention relates to a semiconductor memory device and its data writing method.
2. Related Art
When a specific data is programmed to (written in) a non-volatile semiconductor memory device such as a flash memory and an EEPROM each with a verify function, a data write operation and a data verify operation are repeated a plurality of times. A verify circuit provided in the non-volatile semiconductor memory device checks threshold voltages Vt in a plurality of memory cells and outputs a verify pass signal when each voltage reaches a desired value. When receiving the verify pass signal from the verify circuit, a data write controlling circuit stops the write operation since then. At this time, the data programming to the non-volatile semiconductor memory device ends.
Normally in a non-volatile semiconductor memory device, the data programming ends when the data write/verify operation is repeated approximately three times. In other words, the repetition of data write/verify operation ends before reaching the maximum number of times of data writing (eight times, for example). Conventionally, for this reason, it has been difficult to grasp the period of time needed for testing a non-volatile semiconductor memory device product in repeating the data write operation maximum number of times. Also, it has been actually impossible to estimate the difference of the memory cell characteristics among chips or wafers since each number of times of data write operation until the verify operation passes is different in each memory cell.
In order to solve this problem, there has been already presented a non-volatile semiconductor memory device (JP 11-086575, A) wherein, in the test mode, the data write/verify operation is repeated until reaching maximum number of times of data writing preset in a data write counter circuit. According to this non-volatile semiconductor memory device, when the verify operation passes before reaching maximum number of times of data writing, the data write operation continues to be repeated regardless of this result.
According to the conventional non-volatile semiconductor memory device, however, the data write operation continues to be repeated with the verify operation continued to be repeated even after the verify operation has passed. This state is not preferable in view of saving the time for testing a non-volatile semiconductor memory device product.
The present invention has been achieved in view of aforementioned problems. The object of the present invention is to provide novel and improved semiconductor memory device and its data writing method capable of saving the time for testing to a minimum even in repeating the data write operation maximum number of times.